1) Extent to which a system, optical, acoustic, or electronic, fails to reproduce
accurately at its output the characteristics of the input.
2) Optics: An optical imperfection caused by a
→ lens or → system
of lenses which results in → magnification
differences between different points on the → image.
The points on the → object are misplaced in the image
relative to the → center of the
→ field. See also
→ barrel distortion;
→ pincushion distortion.

The distortion observed in → redshift space of
→ galaxy clusters caused by peculiar velocities of the members
(→ peculiar velocity).
In a perfectly homogeneous → Friedmann-Lemaitre Universe
the redshifts would accurately measure radial distances from the
observer, and the mapping from real space to redshift space would simply be an identity.
In an inhomogeneous Universe the peculiar velocities associated with any inhomogeneous
structure will introduce a distortion in this mapping (N. Kaiser, 1987, MNRAS 227, 1).
See also: → fingers of God,
→ Kaiser effect.